In a CDMA communication system, wideband transmitting signals pass frequency selective fading channel during transmission, thus the identifiable multi-paths may be effectively separated at the receiving end. The optimal receiver may be equivalent as performing maximum ratio combination (i.e., coherent RAKE reception) for multiple of multi-path signals with independent flat fading channel (or frequency nonselective channel) characteristics. Accordingly, the issue of frequency selective channel equalization may be simplified to compensation and diversity reception for multiple frequency nonselective channels. In the case of slow fading channel, a traditional coherent receiver has good receiving function. However, in the case of fast fading due to speeding up of mobile station and increasing of carrier frequency, the mobile channel will introduce strong random interference to transmitting signals both in amplitude and phase, thus it is difficult to calculate the impulse response of channel with ordinary coherent detection techniques in such a case. Therefore, some coherent detection (i.e., channel estimation) methods that may adapt to a large range of channel fading shall be developed to detect channel parameters. In fact, in the core technology of WCDMA (Wideband Code Division Multiple Access), channel estimation technique is the core or foundation of many important techniques, such as coherent RAKE reception, coherent trace for spread spectrum code, Signal to Interference Ratio (SIR) measurement in fast transmitting power control (TPC) or Turbo Code, interference suppression technique, decision and back-substitution-based time domain reference adaptive antenna array, etc.
Traditional techniques for estimating flat fading channel are mainly classified into two categories: techniques (e.g., PSAM, PilotTone) with which to estimate fading with known reference signals; and optimal symbol/sequence estimation techniques that are based on statistical signal processing.
Pilot Symbol Assistant Modulation (PSAM) is used in WCDMA system, and uplink CDMA2000 system. According to the technique, pilot symbols are inserted into data stream periodically to estimate the channel, i.e., it is an interpolation-type channel estimation technique. The insertion frequency of pilot symbols should meet Nyquist sampling rate for fading courses. In the case of low fading rate, PSAM can deliver both good estimation performance and simplicity. FIG. 1 is a structure diagram of a receiver implemented according to above method. Said receiver comprises: pilot frequency/data shunt units 101, . . . 102, channel estimation units 103, . . . 104, multiplier units 105, . . . 106, adder unit 107, output unit 108, and delay units 109, . . . 110. The reciever signals r1 (k) . . . rL(k) enter corresponding pilot frequency/data shunt units 101, . . . 102 respectively; the output pilot symbols are sent to PSAM channel estimation units 103, . . . 104, and the channel information ξd,1 (k), . . . ,.ξd,L (k) corresponding to data code part of every path is calculated with interpolation method and like; data codes output from the other exits of the pilot frequency/data shunt units 101, . . . 102 pass through delay units 109, . . . 110 and then are multiplied in conjugation with above estimated channel values at the multiplier units 105 and 106, and then are added at the adder unit 107, finally, they pass the soft decision output unit 108 to the subsequent decoding part. FIG. 2 shows the Bit Error Rate (BER) curve of a preferable Gauss Second-Order Interpolation Method at several typical Doppler frequencies. It is observed that the methods can satisfy performance requirement in the case of slow fading channel; however, as channel-fading rate accelerates and channel properties vary significantly within just several time slots, such methods can't trace channel variations ideally.
Increasing performance will be required in future mobile communication systems. In particular the higher speed the mobile station has, the severer the variation of channels. In the case of fast fading channels, statistical signal processing-based optimal symbol/sequence estimation technique has obvious advantages. Such methods may be classified into MAP symbol-to-symbol detection techniques and MLSE-based sequence estimation techniques. Though known pilot symbols are needed according to this method, the pilot symbols are only used to provide phase reference necessary for coherent demodulation as well as to prevent accumulative misjudging effect when using decision directed feedback and back-substitution to reduce computing complexity. Therefore, methods like this don't require the insertion frequency of pilot symbols to meet the requirement of Nyquist rate for sampling during fading, thus more accurate channel utilization ratio and channel parameter estimation values at high fading rate can be obtained, compared to PSAM technique. Because that WCDMA delivers 2 GHz RF frequency and supports mobile station with the speed up to 500 Km/s, and the pilot symbols are inserted in at a time slot interval of about 0.667 ms, such a channel estimation technique has to be used. In fact, if the speed of the mobile station is very high, even though the pilot symbols meet the requirement of Nyquist rate, to ensure the interpolation filter has adequate bandwidth and excellent in-band suppression and out-band attenuation, a higher order interpolation filter has to be used, which may result in a long delay and a large amount of calculations. On the other hand, the amount of calculations of the latter method may be reduced enormously with fewer sequence states and reasonable decision and back-substitution. Currently, the Viterbi decoding sequence detection method that utilizes linear prediction filter to estimate channels or the adaptive differentiation-based Viterbi algorithm described in some literatures is an implementation of the techniques. However, linear prediction filter requires prior knowledge about the statistical properties of the channels to calculate its coefficient, which is difficult to do in practice; the adaptive differentiation-based Viterbi algorithm is very sensitive to Doppler frequency expansion, thus the flooring effect of error rate may appear. In conclusion, traditional techniques are difficult to use in real systems. Furthermore, traditional methods consider little about hardware feasibility.
In response to channel estimation for fast fading channel and transmitting sequence detection, the object of the present invention is to provide a practical and feasible method and apparatus for estimating flat fading channels in CDMA system, wherein said method and apparatus is based on optimal sequence estimation for statistical signal processing and may eliminate the influence of fast fading channel.